Heat hardenable compositions comprising epoxide compounds and dicarboxylic anhydrides



HEAT HARDENABLE COMPOSITIONS COMPRIS- ING EPOXIDE COMPOUNDS AND DICARBOX- YLIC ANHYDRIDES Otto Ernst, Pfeflingen, Basel-Land, Switzerland, assignor to Ciba Limited, Basel, Switzerland, a Swiss firm No Drawing. Filed May 15, 1958, Ser. No. 735,370 Claims priority, application Switzerland May 16, 1957 7 Claims. (Cl. 260-47) It is known to use hexachloro-endomethylene-tetrahydrophthalic anhydride for hardening epoxy-resins. This hardener has the advantage of giving short gelatinization times, but is has the disadvantageous property of causing undesired high temperatures during hardening due to exothermic reaction. It is also known to use methylendomethylene-tetrahydrophthalic anhydride as a hardener for epoxy-resins. This hardener produces relatively low exothermic reaction temperatures, but the gelatinization times obtained therewith are for some purposes too long.

It would be expected that the use of a mixture of the two aforesaid anhydrides as a hardener for epoxy-resins would result in gelatinization times and exothermic reaction temperatures between the values observed when each of these anhydrides is used alone.

The present invention is based on the very unexpected observation that, when a mixture of hexachloro-endomethylene tetrahydrophthalic anhydride and methyl-endomethylene-tetrahydrophthalic anhydride is used for hardening epoxy-resins with the addition of an accelerator, exothermic reaction temperatures result which are the same as or lower than the reaction temperature that results when methyl-endomethylene-tetrahydrophthalic an hydride is used alone, notwithstanding that the other component of the mixture, namely hexachloro-endomethylenetetrahydrophthalic anhydride, when used alone results in a considerably higher exothermic reaction temperature than when methyl-endomethylene-tetrahydrophthalic anhydride is used alone.

It is all the more surprising that the gelatinization times that result from using the mixture referred to above are shorter than those resulting from the use of methyl-endomethylene-tetrahydrophthalic anhydride alone. Low exothermic reaction temperatures, notwithstanding shorter gelatinization times, are very desirable, especially in the production of castings because then the cast articles are subject to smaller internal stresses and less shrinkage.

Accordingly, the present invention provides heat hardenable compositions, which comprise an epoxide compound containing n epoxide groups calculated on the average molecular weight of the compound, where n is a whole number or fractional number greater than 1, and a polycarboxylic anhydride mixture comprising hexachloroendomethylenetetraphthalic anhydride and methyl-endomethylene-tetrahydrophthalic anhydride.

The invention also provides a process for hardening epoxide compounds which contain n epoxide groups calculated on the average molecular weight, where n is a whole number or a fractional number greater, than 1, wherein there is used as hardener a polycarboxylic anhydride mixture comprising hexachloro-endomethylene-tetrahydrophthalic anhydride and methyl-endomethylene-tetrahydrophthalic anhydride- In this manner there are obtained self-extinguishing hardened synthetic resin compositions which in' a few cases, as compared with epoxy-resin compositions that have been hardened with hexachloro-endomethylene-tetrahydrophthalic anhydride alone, possess a surprisingly high impact bending strength, for example, amounting to 15-20 centimeters per kilogram per square centimeter. There are advantageously used ashardeners mixtures which are liquid at room temperature and consist of 30-50% by weight of hexachloro-endomethylene-tetrahydrophthalic anhydride and -50% by weight of methyl-endomethylene-tetrahydrophthalic anhydride or 70-50% by weight of a mixture of at least 60% by weight of methyl-endomethylene-tetrahydrophthalic anhydride and at most 40% by weight of endomethylene-tetrahydrophthalic anhydride.

The anhydride mixture is advantageously used in a; proportion such that 0.7-1.1 grams of equivalents, and advantageously 0.8 to0.9 gram equivalents, of anhydride; groups are present per gram equivalent of epoxide groups.

The epoxide compounds used in the invention have a. 1:2-epoxy equivalency greater than 1.0. By the epoxyequivalency reference is made to the average number of. 1:2-epoxy groups. 0

contained in the average molecule of the epoxide com-'- pound. Owing to the usual methods of preparation of the epoxide compounds and the fact that they are ordinarily a mixture of chemical compounds having somewhat different molecular weights and contain some com-- popnds wherein the terminal epoxy groups are in hydrated; form, the epoxy equivalency of polyepoxycompounds is: not necessarily an integer of at least 2, but in all cases it; is a value greater than 1.0. As epoxide compounds of the kind defined above, there may be mentioned, for example, epoxidized diolefines dienes or cyclic dienes, such as butadiene oxide 1:2:5z6-diepoxy-hexane and 1:2:4:S-diepoxy-cyclohexane; epoxi:

mentioned basic polyepoxide compounds, such as areob tained by the reaction of a secondary aromatic diamine, such as 4:4'-di-[monomethylamino]-diphenylmethane,

with epichlorhydrin in the presence of an alkali.

There may also be used polyglycidyl esters suchas are otbainable by the reaction of a dicarboxylic acid with'epichlorhydrin or dichlorhydrin in the presence of an alkali, Such polyesters may be derived from aliphatic dicarboxylic acids, such as oxalic acid, succinic acid, glutaric' acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, and especially aromatic dicarboxylic acids, such as phthalic acid, isophthalic acid, terephthalic acid, 2:6.-naph-- thylene-dicarboxylic acid, diphenyl-ortho:ortho-dicarboxylic acid, ethylene glycol bis-(para-carboxyphenylfi ether or the like. There may be mentioned, for example, diglycidyl adipate and diglycidyl phthalate, and also diglycidyl esters which correspond to the average formula- 2,918,435 l aterited Apr-.4, .1961 a in which X represents an aromatic hydrocarbon radical,

such as a phenyl group, and Z represents a small whole number or a small fractional number.

There may also be mentioned polyglycidyl ethers such as are obtainable by the etherification of dihydric or polyhydric alcohol or a diphenol or polyphenol with epichlorhydrin or dichlorhydrin in the presence of an alkali. These compounds may be derived from glycols, such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol-1:2, propylene glycol-1:3, butylene glycol-1:4, pentane-lzS-diol, hexane-1:6-diol, hexane-2:4:6-triol, glycerine and especially diphenols or polyphenols, such as resorcinol, pyrocatechol, hydroquinone, 1 :4-dioxynaphthnlene, 1:5-dioxynaphthalene, phenol-formaldehyde condensation products, bis-[4-hydroxyphenyl1-methane, bis-[4-hydroxyphenyl] methylphenyl methane, bis- [4-hydroxyphenyl]-tolyl-methane, 4: 4-dioxydiphenyl, bis- [4-hydroxyphenyl] -sulfone and especially 2:2-bis-[4-hydroxyphenyl1-propane. There may be mentioned ethylene glycol diglycidyl ether and resorcinol diglycidyl ether, and also diglycidyl ethers which correspond to the average formula hydrides or mixtures of anhydrides of the polycarboxylic acids, with the use of 0.9 gram equivalent of anhydride groups for every gram equivalent of epoxide group present in the epoxy resin. To compare the gelatinization times and the maximum exothermic reaction temperatures 10 specimens of casting resin are made with the use of 100 grams of casting resin mixture for every specimen and these specimens are then placed in an oven heated at 120 C.

In the case of specimens 1, 3, 4 and 5and 1a, 3a, 4a and 5a respectively-the epoxy resin, if desired together with the accelerator, and the anhydride or anhydride mixture are separately heated to 120 C., then combined and stirred; this point of time is designated as time 0 Mi. In the case of specimens 2 and 2a respectively the epoxy resin, if desired together with the accelerator, is heated to 150 C., the solid hexachloroendomethylcnertetrahydrophthalic anhydride is added and dissolved at 120 C.; this point of time is designated in this case as time 0 Min. As accelerator is further added to specimens la-Sa 0.5 part of tris-(dimethylaminomethyl)-phenol.

in which X represents an aromatic radical, and Z represents a small whole number or small fractional number.

There are especially suitable epoxy resins that are liquid at room temperature, for example, those obtained from 4:4-dioxydiphenyl-dimethylmethane, which have anepoxide content of about 3.8 to 5.88 epoxide equivalents per kilogram. Such epoxy-resins correspond, for example, to the average formula in which Z represents a small whole number or small fractional number, for example, between 0 and 2.

Alternatively, molten solid epoxy-resins or solutions of solid epoxy-resins may be used. The mixtures of epoxy-resin and anhydrides or their components may be mixed with a diluent, plasticiser, solvent, pigment, filling material and/or hardening accelerator. If desired, they may also contain a reactive diluent and/or plasticiser.

The compositions containing an epoxy-resin (polyepoxide compound) and a polycarboxylic anhydride mix ture as defined above may be used, for example, as casting, coating, dipping or laminating compositions, as adhesives or impregnating agents, as resins for the manufacture of tools and patterns and as putty-like or modelling compositions.

The following examples illustrate the invention, the parts and percentages being by weight:

Example 1 For the manufacture of casting resin mixtures an epoxy resin, which is liquid at room temperature and has an epoxide content of at least 5.1 gram equivalents of epoxide groups per kilogram, which has been prepared by condensing 1 mol of 4:4'-dihydroxydiphenyl dimethylmethane with at least 2 molecular proportions of epichlorohydrin in the presence of an aqueous sodium hydroxide solution, is mixed with the under-mentioned an- The abbreviations used in the following table have the following meaning:

Gclatinl- Maximum Composition of polycar Accelzation exothermic Specimen boxylic acid anhydride erator time in reaction ruins. at tempora- 120 C. ture 111 C.

1 60% l\1ET+40% HET 0 50 143 2 HET v 0 ml 101 3 100% MET 0 no gelatinizutlou after 12 ours 100% PILA 0 85 l 105 50%7MET-i-40'z, HET+ 0 50 t 0 i 60% MET-P107}; HET..- 0. 5 0 i 101 100% HEI- 0. 5 3 210 100% M ET- 0. 5 14 161 100% PlLA 0. 5 4 210 50% MET-+40% IIET+ 0. 5 6 161 As can be seen from this table the specimens 1 and 1a according to the invention surprisingly give the lowest exothermic reactions at practically identical gelatinization times.

Example 2 70 Degreased and ground sheets of aluminum (x25 x 75 shear strengths of the laminations are measured.

and specimen 2 and 3 like specimens 2,3, 4, 5, 7 and Tensile 8 I mple 3, Specimen Tested at shear 7 0. strengthln I kg./sq.mm. .7 Gelatiniza- Maximum 5 tion time exothermic Composition of the polyat an oven reaction -40 2. Specimen carboxylic acid anhydride temperatempera- 40 1. 3 ture of ture in 40 1.8 120 C. C. +23 2. 25 inmins. 23 1 6% 525 1.55 100% BE 1 190 +125 1.05 60% MET+40% HET 2 '155 +125 0.85 100% MET 4 180 +150 0.85 +150 0.9 Example By alkaline condensation of resorcmol and epichloro- As the above table shows, the specimen 1a according hydrin a liquid epoxy resin is prepared whlch has an to the invention surprisingly has substantially better, or at least equal, tensile shear strength.

Example 3 A novolak prepared from 2 molecular proportions of orthocresol and 1 molecular proportion of formaldehyde in the presence of dilute hydrochloric acid is condensed with epichlorohydrin in the presence of sodium hydroxide. After having been washed with water until it is free from salt and distilled in a water-jet vacuum up to 150 C., the epoxy resin is liquid at room temperature and has an epoxide content of 4.9 gram equivalents of epoxide groups per kilogram. Casting resins are prepared by mixing this epoxy resin with the under-mentioned anhydrides or mixtures of anhydrides of polycarboxylic acids and amine accelerators, the addition amounting to 0.8 gram equivalent of anhydride groups for every gram equivalent of epoxide group. To determine the gelatinization times and the maximum exothermic reaction temperatures 8 specimens of 100 grams each of casting resin are prepared. In the case of specimens 1 and epoxide content of 8.2' gram equivalents of epoxide of epoxide group. There are further added 0.5%, cal

culated from the weight of the casting resin mixture, of tris-(dimethylamino-methyl)-phenol as accelerator. Y To determine the gelatinization times and the maximum exothermic temperatures 3 specimens of 100 grams each of casting resin are made. In the case of specimen 1 the epoxy resin is heated with the amine accelerator to 150 C, the solid hexachloroendomethylenetetrahydrophthalic anhydride is added and dissolved at 120 C. In the case of specimens 2 and 3 the epoxy resin with the amine accelerator on the one hand the anhydride or inixt'ure' of anhydrides of polycarboxylifc acids onthe other hand, are separately heated to 120 C. and then onlymixed together. H

6 the epoxy resin together with the amine accelerator is heated to 150 C., the solid hexachloroendomethylene tetrahydrophthalic anhydride is added and dissolved at Composition of the polyat an oven reaction 120 0. In the case of Specimens 2, 3', 4, 5, 7 and 8 the 40 Specimen V carboxylm W anhydnde $3 3 5 a r I lift? 1H epoxy resin with the amine accelerator, and the polycar- 120 0. 0. boxylic acid anhydride or mixture of anhydrides ofpolymmmscarboxylic acids are separately heated to 120 C., then 1007 H x .90

v I combined and mixed. a %"MET+40% HETn" 2 M2163 Example 4 MET. 4 19; v V A novolak is made from 2 molecular proportions of Gelatiniza Maximum tion time exothermic Specimen Composition of the poly- Amine accelerator at an oven reaction carboxylic acid anhydride temperatern eratureof ture O. C.inmius.

100% HE'r 2 191 60% MET+40%HET.. 0.5% oftris-(di-' a 158 100% MET methylamino- 11 164 50% M E'1+50% HET methyl) phenol. 2 162 sesa e t as:

0.5% (limethylamiiSZa liE EiPftET-::::i 2 at phenol and 1.1 molecular proportion of formaldehyde 7 Example 6 V a I in the presence of dilute hydrochloric acid and from it 7 A epoxy, resin i prepared b condensation of 1:4-

an epoxy resin is prepared as described in Example 3, which is liquid at room temperature and has an epoxide content of 5.l gram equivalents of epoxide groups per kilogram. fBy mixing this epoxy resin with the undermentioned anhydrides or mixtures of anhydrides of poly: carboxylic acids casting resins are prepared, 0.9 gram equivalent of anhydride groups being'added for every gram equivalent of epoxide group. There are further added 0.5%, calculated from the weight of the casting resin mixture, of tris-(dimethylaminomethyl)-phenol as accelerator. To determine the gelatinization times and the maximum exothermic reaction temperatures 3 speci-, mens of 100 grams each of casting resin are made, speci-. men 1 being mixed like specimens 1 and'6 in Example 3,

butanediol and epichlorohydrin in the presence of-stannic chloride and subsequent treatment of the condensate with. alkali. The resulting epoxy resin has an epoxide content of 7.8 gram equivalents of epoxide groups per kg., and by mixing this epoxy resin with the under-mentioned anhydrides of polycarboxylic acids and amine accelerators casting resins are made. Furthermore, there are added 0.5%, calculated from the weight of the casting resin mixture, of tris-(dimethylamino-methyl)-phenol as celerator. To determine the gelatinizationttimcs'andthe g maximum exothermic reaction temperatures 7 specimens 7 of 100 grams each of casting resin are made; Specimen 1 is mixed like specimen 1 in Example 5, andspecimenl 2 to 7 like specimens 2 and 3 in Example 5.

By alkaline treatment of a condensation product from 1 molecular proportion of aniline and at least 2 molecular proportions of epichlorohydrin a liquid epoxy resin is prepared having an epoxide content of 6.7 gram equivalents of epoxide groups per kilogram. By mixing this epoxy resin with the under mentioned anhydrides or mixtures of anhydrides of polycarboxylic acids and amine accelerators casting resins are made, 1.0 gram equivalent of anhydride groups being added for every gram equivalent of epoxide group. There are further added 0.1% calculated from the weight of the casting resin mixture, of tris-(dimethylaminomethyl)-phenol as accelerator. To determine the gelatinization times and the maximum exothermic reaction temperatures 3 specimens of 100 grams each of casting resin are prepared. Specimen 1 is mixed like specimen 1 in Example 5, and specimens 2 and 3 like specimens 2 and 3 in Example 5.

Example 8 By condensing 4:4'-dihydroxyphenyldimethylmethanc with epichlorohydrin in the presence of aqueous sodium hydroxide solution an epoxy resin is prepared which is solid at room temperature and has an epoxide content of 2.5 gram equivalents of epoxide groups per kilogram. By mixing this epoxy resin with the under-mentioned anhydn'des or mixtures of anhydrides of polycarboxylic acids and amine accelerators casting resins are made, 0.9 gram equivalent of anhydride groups being added for every gram equivalent of epoxide group. To determine the gelatinization times and the maximum exothermic reaction temperatures 3 specimens of 100 grams each of casting resin are prepared. In the case of specimen '1 the epoxy resin is fused and heated to 150 C., 0.25% of tris(dimethylaminomethyl)-phenol are stirred in, and the solid hexachloroendomethylenetetrahydrophthalic anhydride is then added and dissolved at 120 C. In the case of specimens 2 and 3 the epoxy resin and the polycarboxylic acid anhydride or mixture of anhydrides of polycarboxylic acids are separately'heated to 120 C., 0.25% of tris-(dimethylaminomethyl)-phenol are stirred into the epoxy resin, and the whole is then mixed with the polycarboxylic acid anhydride or mixture of anhydrides of polycarboxylic acids. To determine the gelatinization times and the maximum exothermic reaction temperatures an amount of 100 grams is used in each ,case.

Gelatiniza Maximum tion time exothermic Composition of the polyat an oven reaction Specimen carboxylic acid anhydride tcmpcratemperature ol turn in 0 C. in ruins 100% REF 1 60% MET+-i0% HEI 3 137 100% MET 14 143 Example 9 By condensing 1:5-dihydroxynaphthalene with epichlorohydrin in the presence of alkali in epoxy resin is prepared which is solid at room temperature and contains 3.80 equivalents of epoxide groups per kilogram. By mixing this epoxy resin with the under-mentioned anhydrides or mixtures of anhydrides of polycarboxylic acids and amine accelerators casting resins are prepared, 0.9 gram equivalent of anhydride groups being added for every gram equivalent of epoxide group. There are further added 0.25%, calculated from the weight of the casting resin mixture, of tris-(dimethylaminomethyl)- phenol as accelerator. To determine the gelatinization times and the maximum exothermic reaction temperatures 3 specimens of 100 grams each of casting resin are prepared. Specimen 1 is mixed like specimen 1 in Example 8, and specimens 2 and 3 like specimens 2 and 3 in Example 8.

Example 10 Casting resins are prepared by mixing an epoxy resin, which has been obtained by alkaline condensation of terephthalic acid and epichlorohydrin (Metallon 130, makers: Messrs. Henkel) and contains 3.3 gram equivalents of epoxide groups per kilogram, with the undermentioned anhydrides or mixtures of anhydrides of polycarboxylic acids, 1.0 gram equivalent of anhydride groups for every gram equivalent of epoxide group being added. There are further added 0.25 calculated from the weight of the casting resin mixture, of tris-(dimethylaminomethyl)-phenol as accelerator. To determine the gelatinization times and the maximum exothermic reaction temperatures 3 specimens of 100 grams each of casting resin are prepared. Specimen 1 is mixed like specimen 1 in Example 8, and specimens 2 and 3 like specimens 2 and 3 in Example 8.

Gclatinlza- Maximum tlon time exothermic Composition of the polyat an oven reaction Specimen oarboxylic acid anhydride tempera temperature of ture in 120 C C. in mins 100% HET 1 60% MET+40% HET 1 171 100% MET 3 176 What is claimed is:

V l. A heat-hardenable composition which comprises a 1:2-epoxide compound having a 1:2-epoxy equivalency greater than 1.0 and as hardening agent theretor a mixture comprising 30-50% by weight of said mixture of hexachloroendomethylene-tetrahydrophthalic anhydride and 70-50% by weight of said mixture of methylendomethylene-tetrahydrophthalic anhydride, said mixture be- 9 ing employed in a proportion such that 0.7-1.1 gram of equivalents of anhydride groups are present per gram equivalent of epoxide groups.

2. A heat-hardenable composition which comprises a 1:2 epoxide compound having a 1:2-epoxy equivalency greater than 1.0 and as hardening agent therefor a mixture comprising 30-50% by weight of said mixture of hexacbloroendomethylene-tetrahydrophthalic anhydride and 70-50% by weight of said mixture of methylendomethylene-tetrahydrophthalic anhydride, which mixture is liquid at room temperature, and is employed in a proportion such that 0.7-1.1 gram of equivalents of anhydride groups are present per gram equivalent of epoxide groups.

3. A heat-hardenable composition which comprises a 1:2-epoxide compound having a 1:2-epoxy equivalency greater than 1.0 and as hardening agent therefor a mixture consisting of 30-50% by weight of said mixture of hexachloroendornethylene-tetrahydrophthalic anhydride and 70-50% by weight of said mixture of methyl-endomethylene-tetrahydrophthalic anhydride, said mixture being employed in a proportion such that 0.7-1.l gram of equivalents of anhydride groups are present per gram equivalent of epoxide groups.

4. A heat-hardenable composition which comprises a 1.2-epoxide compound having a 1:2-epoxy equivalency greater than 1.0 and as hardening agent therefor a mixture consisting of 30-50% by weight of said mixture of hexachloroendomethylene-tetrahydrophthalic anhydride and 70-50% by weight of said mixture of a mixture of at least 60% by weight of methylenedomethylene-tetrahydrophthalic anhydride and at most 40% by Weight of endomethylene-tetrahydrophthalic anhydride, said total mixture of anhydrides being employed in a proportion such that 0.7-1.1 gram of equivalents of anhydride groups are present per gram equivalent of epoxide groups.

10 5. A heat-hardenable composition which comprises a polyglycidy-l ether of a polyhydric phenol and as hardening agent therefor a mixture consisting of 30-50% by weight of said mixture of hexachloroendomethylene-.

tetrahydrophthal-ic anhydride and -50% by weight of said mixture of methyl-endomethylene-tetrahydrophthalic anhydride, said mixture being employed in a proportion such that 0.7-1.1 gram of equivalents of anhydride groups are present per gram equivalent of epoxide groups.

6. A heat-hardenable composition which comprises a polyglycidyl ether of 4:4'-dihydroxy-diphenyl-dimethylmethane and as hardening agent therefor a mixture consisting of 30-50% by weight of said mixture of hexachloroendomethylene-tetrahydrophthalic anhydride and 70-50% by weight of methyl-endomethylene-tetrahydrophthalic anhydride, said mixture being employed in a proportion such that 0.7-l.1 gram of equivalents of anhydride groups are present per gram equivalent of epoxide groups.

7. A heat-hardenable composition as claimed in claim 6, wherein the polyglycidyl ether is liquid at room temperature and contains about 3.8 to 5.88 gram equivalents of epoxide groups per kilogram.

References Cited in the file of this patent UNITED STATES PATENTS 2,744,845 Rudoff May 8, 1956 2,768,153 Shokal Oct. 23, 1956 2,863,853 Pschorr Dec. 9, 1958 2,934,520 Maywrnik Apr. 26, 1960 OTHER REFERENCES Weiss: Ind. Eng. Chem. vol. 49, pp. 1089-1090.

UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent N0. 2 978,435 April 4 1961 Otto Ernst It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 39 after "diolefines" insert a comma; column 5, lines 45 and 46, strike out Example 4 A novolak .is made from 2 molecular proportions of and insert the same above "phenol and 1.1 molecular proportion of formaldehyde" in line 60, same column; column 8, line 14, for "in", second occurrence, read an Signed and sealed this 29th day of August 1961.

(SEAL) .Att'est;

DAVID L. LADD Commissioner of Patents ERNEST w SWIDER Attesting Officer 

1. A HEAT-HARDENABLE COMPOSITION WHICH COMPRISES A 1:2-EPOXIDE COMPOUND HAVING A 1:2-EPOXY EQUIVALENCY GREATER THAN 1.0 AND AS HARDENING AGENT THEREFOR A MIXTURE COMPRISING 30-50% BY WEIGHT OF SAID MIXTURE OF HEXACHLOROENDOMETHYLENE-TETRAHYDROPHTHALIC ANHYDRIDE AND 70-50% BY WEIGHT OF SAID MIXTURE OF METHYLENDOMETHYLENE-TETRAHYDROPHTHALIC ANHYDRIDE, SAID MIXTURE BEING EMPOLYED IN A PROPORTION SUCH THAT 0.7-1.1 GRAM OF EQUIVALENTS OF ANHYDRIDE GROUPS ARE PRESENT PER GRAM EQUIVALENT OF EPOXIDE GROUPS. 